The identification of a new pregnancy-supporting hormone in horses has resolved a reproductive mystery that has puzzled scientists for decades, and it could have important implications for sustaining human pregnancies, reports a team of researchers, led by a University of California, Davis (UC Davis) veterinary scientist.

“This work ends 50 years of speculation as to how horses sustain the last half of their pregnancies, despite the fact that the hormone progesterone is no longer detectable in blood,” said Alan Conley, BVSc, MS, PhD, a reproductive physiologist in the School of Veterinary Medicine and senior author on the study.

“We show for the first time that in horses this ‘new’ progestin, DHP (or dihydroprogesterone), is equally effective as progesterone in sustaining pregnancy during the last few months,” he said.

The role of progesterone is so important for a successful pregnancy that reproductive biologists have widely accepted for more than 80 years that pregnancies in humans and other mammals could not be carried to term without it